The present invention relates to degassers on worm-type melting and conveying devices of plastic injection molding machines in general and, in particular, to a new and useful degasser having a vent pin arranged in the degassing zone of a melting cylinder of the melting and conveying device adjoining a shearing zone in the direction of conveyance, which pin has a widened charging pocket relative to the cylinder bore which opens to the atmosphere over a pin opening.
The cylinder zones on both sides of such a degassing zone are provided, in a known manner, with heating jackets which ensure, on the one hand, the proper melting of the granulated plastic supplied to the zones preceding the degassing zone and, on the other hand, keep the melted plastic following the degassing zone at a temperature required for an injection molding operation.
The so-called shearing zone forms an area in the melting cylinder, with a relatively small flow volume, while the adjoining degassing zone represents a considerable volume increase. The plasticized material passing through the shearing zone under a relatively high pressure can expand in the degassing zone over the charging pocket and the pin bore, to a lower pressure, that is, to ambient pressure.
In plasticizing hygroscopic plastics, such as acrylonitrile-butadiene-styrene, polycarbonate, or particularly polyamides, for example, PA 6 and PA 6.6, moisture and other volatile substances will diffuse toward the surface of the melt, depending on the moisture content and the cylinder temperatures generated by the heating jackets, due to the pressure gradient between the shearing zone and the degassing zone. From there, such substances can escape through the pin bore.
It was found, in practice, however, that the charging pocket, which should permit easy escape of the gas, due to increased volume, is easily clogged, since the entering material can at least partly solidify in the pin vent arranged in an unheated zone, due to large heat dissipation in this area. The receiving volume of the pocket is thus reduced, which can lead to the issuance of plastic material through the pin bore, particularly with highly hygroscopic material, due to excessive swelling. Excessive swelling could naturally also be achieved by reducing the temperature in the zone preceding the degassing zone. However, this would prevent the complete diffusion of vapor and gases. Particularly when changing the machine over to a different injection material, temporary solidification of material in the pocket is unavoidable, and there is no assurance that the material solidified in the pocket will melt again when the machine is started again.